Interleukin-6 (IL-6), a multifunctional cytokine produced in monocytes, fibroblasts, endothelial cells, and keratinocytes, is induced by a variety of stimulating signals, including lipopolysaccharide (LPS), poly,(I), poly (C), IL-1, tumor necrosis factor (TNF), and platelet-derived growth factor. Some of these signals induce IL-.6 effectively only in one cell type, and this selectivity of induction may explain selectivity of biologic effects. In the present study, we show that IL-1 beta,.previously known to be a potent inducer of IL-6 in fibroblasts, endothelial cells, and keratinocytes, but not in monocytes, is also a potent inducer of IL-6 in peripheral blood monocytes. High level IL-6 activity that could be neutralized by specific antibodies to IL-6 was detected in supernatants of IL-1-stimulated monocytes. Maximal induction required IL-1 concentrations of 10 ng/ml. As judged by sodium dodecyl sulfate-polyacrylamide gel electrophoresis under reducing conditions, IL-6 species of relative molecular mass of 19 to 26 Kd could be specifically immunoprecipitated from supernatants of IL-1-induced monocytes. Size heterogeneity is a reported feature of IL-6 produced in a variety of cell types, and monocyte-derived IL6 induced by either IL-1 or LPS displayed similar size heterogeneity. The highly purified recombinant IL-1 beta preparation used contained little, if any, LPS. In addition, monocyte production of IL-6, induced by IL-1 beta, was specifically neutralized by anti-IL-1 beta antibodies, demonstrating that IL-1, rather than a contaminant in the IL-1 preparation, was responsible for IL6 induction. A number of biologic activities have been ascribed both to IL-1 and IL-6. The finding that IL-1 induced IL-6 in monocytes may help in defining the spectrum of biologic activities of each of these interactive cytokines.